Biogen Idec Inc. and Elan Corp. said Wednesday that they have asked US and European regulators to approve their Tysabri drug for a wider universe of multiple sclerosis patients.

Because of a possible risk of contracting a rare brain disease that is sometimes fatal, Tysabri is generally recommended for US patients who are not responding to or cannot tolerate an alternative MS therapy.

Weston-based Biogen Idec said in a press release that it is asking the Food and Drug Administration to approve Tysabri as an initial treatment for many MS patients.

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MS patients are screened to detect the presence of certain antibodies for something called the JC virus.

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Biogen Idec and Elan, its Irish partner, are asking the FDA to approve Tysabri as a first-line MS treatment for US patients who test negative for antibodies to the JC virus.

Biogen Idec declined to quantify how many additional patients might benefit if regulators approved its request for Tysabri’s wider use, and the company would not comment on how approvals might affect its revenues.

In the third quarter, Tysabri had global sales of $404 million, Biogen Idec reported. That amount was shared between Biogen Idec and Elan. Biogen Idec’s share was $275 million. Biogen Idec’s total third-quarter revenue was $1.4 billion.

When it issued third-quarter results in late October, Biogen Idec estimated that 71,100 patients were on commercial and clinical Tysabri therapy worldwide.

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In a statement Wednesday, Biogen Idec chief medical officer Dr. Alfred Sandrock, said: “Many appropriate patients are already benefiting from Tysabri. A first line approval would allow people with MS access to a highly efficacious treatment earlier in the course of the disease, potentially leading to better outcomes.”